Having Level 30 wizards is no different than the existance of nuclear weapons in a modern setting. They do have the potential to potentially end every battle before it begins, but you don't throw them at every little problem all the time.
They need to know what problems exist, they need to know which ones require their personal intervention, they need to be ready when something really important happens, and they also need to keep in mind how much attention their personal presence draws. Because there are as many evil epic spellcasters as there a good ones.
When you have small and medium scale problems, and lots of low and mid level heroes in the area, then let those heroes deal with the problems. How many people have actually played campaigns in which the PCs where dealing with enemies so powerful that the top 10 good guys would consider it the greatest threat in the whole world at that time? If they have to take care of the problem, then it means there are no local heroes able to deal with it. In which case there is no problem, since the PCs would not have been able to do anything about it in the first place.